Common photoreceptors for use in electrophotography are inorganic light-sensitive materials such as amorphous selenium, selenium compounds, cadmium sulfide and zinc oxide, as well as organic compounds typified by poly(vinylcarbazole) and poly(vinylcarbazole) derivatives.
As is well known, amorphous selenium and selenium alloys have excellent properties for use as electrophotographic photoreceptors and are commercially used. However, their manufacture requires the complex step of vapor deposition, and the vapor deposited film has no flexibility. Zinc oxide as a photoreceptor material is dispersed in a resin to prepare a disperse light-sensitive composition, but this has low mechanical strength and is not suitable for cyclic use.
Poly(vinylcarbazole) widely known as an organic photoconductive material is advantageous with respect to transparancy, film-forming properties and flexibility. However, poly(vinylcarbazole) per se is insensitive to light in the visible range and cannot be immediately used as a photoreceptor. Various methods have been proposed for sensitizing the poly(vinylcarbazole). THe use of a sensitizing dye is effective in extending the spectral sensitivity of the poly(vinylcarbazole) to the visible range, but the sensitivity of the so treated polyvinyl carbazole is still insufficient for use as an electrophotographic photoreceptor and it undergoes significant fatigue under light illumination. When poly(vinylcarbazole) is chemically sensitized with an electron acceptor compound, sufficient sensitivity for use as an electrophotographic photoreceptor is obtained. Several of the so treated poly(vinylcarbazole) compounds are commercially used, but their mechanical strength and service life are still unsatisfactory.
Among photoreceptors having sensitivity to light in the longer wavelength region are inorganic compounds such as Se/Te, Se/As and CdSe. Many of these inorganic compounds are designated as poisons, dangerous drugs or special chemical substances under regulations that require utmost care in handling, particularly in disposal. Futhermore, these compounds are difficult to manufacture and their production cost is high. As another disadvantage, they have no flexibility and can only be shaped into a belt form with great difficulty.
Energetic efforts have been made in the study of electrophotographic photoreceptors using organic disperse photosensitive materials wherein a charge generation layer and a charge transport layer are laminated on an electrically conductive support. For example, U.S. Pat. No. 4,108,953 discloses an electrophotographic photoreceptor having a charge generation layer containing an organic photoconductive materials such as phthalocyanine type pigments, quinacridones, etc. and a charge transport layer having a specific aromatic diamine dispersed in a binder (hereafter referred to as "double-layered photoreceptor"). Double-layered photoreceptors using other organic disperse photosensitive materials are also described in U.S. Pat. Nos. 4,050,935, 4,127,412, 4,173,472, 4,175,960 and 4,284,699. Several authors have reported that phthalocyanine in the form of a disperse photosensitive material exhibits excellent electrophotographic characteristics (U.S. Pat. Nos. 4,018,953 and 4,181,772). Organic phthalocyanine compounds are non-toxic and inexpensive, but they have low sensitivity to light below 550 nm and are difficult to purify. No practical phthalocyanine compound has been found that exhibits flat photosensitivity characteristics over the visible to near infrared region.
U.S. Pat. No. 3,824,099 discloses a double-layered photoreceptor having a charge generation layer containing squaric acid pigments dispersed in a binder and a charge transport layer of tri-aryl pyrazoline, and U.S. Pat. No. 4,123,270 discloses a photoreceptor having a charge generation layer containing derivatives of squaric acid pigments which are soluble in an organic primary amine. The squaric acid pigments used in these U.S. Patents are represented by the formula ##STR2## and Z is a hydrogen atom, a hydroxy group or a methyl group. However, these squaric acid pigments still have defects in that charge retention is low and residual potential is high, resulting in low image density and high fog density. Further, the squaric acid pigments do not exhibit flat photosensitivity characteristics over the visible to near infrared region.